Microwave Measurement System for Characterizing Liquid Samples Based on a Modified HMSIW

A microwave measurement system for retrieving complex permittivity of liquid sample based on the modified half-mode substrate integrated waveguide (HMSIW) is proposed in this article. The proposed sensing system consists of the passive resonant sensor and the RF circuits. Meanwhile, the passive sensor is evolved from the traditional full-mode SIW, the HMSIW is obtained by cutting along the magnetic wall, and the resonant frequency for TE101 is almost unchanged. By adding a serrated interdigital structure on the top surface of HMSIW, the confinement of the electric field is enhanced and it is beneficial to improve the sensitivity. In addition, the resonant frequency of TE101 shifts toward the low-frequency band, which is beneficial to reduce the electrical size of the passive sensor. The RF circuits are composed of a power divider and a gain/phase detector; it should be noted that the detector can output two dc voltages, i.e., V-mag and V-phs. As liquid samples with different complex permittivity are loaded at the sensing area, V-mag and V-phs will have different output dc voltages. According to the relationship among complex permittivity of liquid samples, Vmag and Vphs, the mathematical models can be established to predict the unknown liquid samples. In measurement, the maximum errors for extracting real and imaginary parts of complex permittivity are -3.84% and 11.72%, respectively. The proposed sensing system has the advantages of discarding the dependence on an expensive vector network analyzer (VNA) and lower the cost.